Over the years, optical fiber transmission systems have increased in capacity from several megabits per second (Mb/s) to 2.5 gigabits per second (Gb/s) or higher. Operating at a desired level of bit-error rate (BER) depends in part on connections along an optical path and the strength of received signals. Another consideration is BER degradation attributed to reflections in the optical path. A series of reflection points can generate multiple reflections among themselves thereby worsening the degradation. This is particularly important in high speed lightwave transmission systems (over 1.0 Gb/s) and amplitude modulated (AM) cable television (CATV).
A very much used ferrule connector for terminating and connecting two optical fibers is one which is referred to as an ST.RTM. connector, ST being a registered trademark of AT&T. The ST.RTM. connector is disclosed, for example, in U.S. Pat. No. 4,934,785 which issued on June 19, 1990 in the names of T. D. Mathis and Calvin M. Miller.
An ST connector includes a cylindrical plug or ferrule, as it is often called, having a passageway therethrough for receiving an end portion of an optical fiber to be terminated. The plug which is received in a cap is spring-loaded. When two of the plugs are received end-to-end in a coupling sleeve, one or both of the plugs bodies is moved along its longitudinal axis to make the connection.
Connections between optical fiber ends require great care. Because the core diameter of optical fiber may be as small as 8 microns, it is difficult to align precisely cores of two optical fibers to be connected to achieve tolerable losses. Not only do the cores of the end portions of two optical fibers to be connected need to be aligned, but also the axes of the optical fiber end portions must be parallel.
Additional concerns must be addressed by optical connection arrangements. Often, attenuators are needed in the transmission path to reduce the strength of an incoming signal to a required level. Many optical fiber communication systems require a method of decreasing optical power at a reducing station to avoid the saturation of receivers. Such a reduction in power may be accomplished by introducing into the system a device known as an attenuator which is designed to dissipate or to attenuate a controlled fraction of the input power while allowing the balance to continue through the system.
Changing of the attenuation level also may be required. It is known that the efficiency of a circuit decreases with age and that the power of a signal source which may be adequate at the beginning of life of a circuit later may become inadequate. If the power of the signal at the beginning is chosen so that it remains adequate later, components of the circuit may become saturated early in life. Additionally, the unearthing of cable which results in repairs that add optical loss to the transmission path can be compensated for with a lower loss attenuator. Often times, the required attenuation is induced at a patch panel or at an optical backplane where it is most convenient to insert an attenuator between connectors.
Attenuators for biconic optical connectors are available commercially in various configurations. A biconic connection arrangement generally includes two tapered plugs each terminating an optical fiber with the plugs being received in a sleeve having opposing conically shaped cavities. Typically, prior art in-line biconic attenuators are non-contacting, that is, they are not contacted by the fiber ends in the biconic connector plugs. However, in a recently issued patent, U.S. Pat. No. 4,900,124 which issued on Feb. 13, 1990 in the names of N. R. Lampert, et al., end pedestals of two biconic connector plugs do engage an attenuator element.
Prior art fixed attenuators generally fall into four classes. First, there is an air gap attenuator with various fixed filter elements suspended in the air gap and in which a coupling sleeve includes means for preventing contact between two fiber ends or with the various filter elements suspended in the air gap. A second class design is one which comprises an air gap in which attenuation is increased by increasing the gap. In a third type, a high density, translucent, laminated element that varies in the thickness of a carbon layer thereof for different attenuation levels is mounted in a transverse slot. In one such design, an alignment sleeve includes an attenuating element capable of transverse movement in the alignment sleeve. See, for example, U.S. Pat. No. 4,717,234 which issued on Jan. 5, 1988 in the names of R. W. Barlow, et al. Such a design is intended primarily for multimode-to-multimode connections. A fourth class of attenuator for use in a biconic arrangement includes an index matched spacer. None of these appear to be suitable for use with cylindrical ferrule, single mode to single mode connecting arrangements where low reflectance is important.
Complicating matters for attenuating cylindrical ferrule connections is the recognition that there is no universally accepted ferrule connector. AT&T's ST connector is used widely, as is a Japanese based connector referred to as the FC connector. Desirably, the sought after attenuator may be used for either connector. What is needed and what does not seem to be available is an attenuator which may be used in single mode to single mode ferrule type connecting arrangements, such as for example, ST connector to ST connector, ST connector to FC connector or FC connector to FC connector.
Another problem in arriving at an attenuator having the sought-after features is that during the connection process for the ST connector, movement occurs in a plug which is first inserted into a coupling sleeve when another plug is inserted into the sleeve. Any widely accepted attenuator system must be able to accommodate such movement.
It appears that a widely acceptable in-line attenuator for an ST connector or for the FC connector is not yet available. Attempts have been made to use an air gap attenuator or a membrane material such as carbon Mylar.RTM. plastic in a transverse slot. Materials used for the attenuator have had a somewhat rough contacting surface on a micrographic scale. Those attenuators which have been tried for the ST and FC connectors have not both been capable of low reflection connections.
The use of an attenuator in an optical path also raises a concern about high reflections and reflected power for systems operating above 1.0 Gb/s. High bit rate systems have been plagued by high reflective loss from attenuators that vary either in the length of the air gap or in carbon density.
A significant need arises for a low reflection attenuator that can be used in single mode-to-single mode connections of ST and/or FC connectors in optical systems that operate above 1.0 Gb/s. Light which is reflected from components such as connectors and splices along a fiber link can strike a source of light such as a laser, for example, which may affect adversely the performance of the laser. Optical power fluctuation, pulse distortion and phase noise may result. Also affected adversely may be the wavelength, linewidth and threshold current of laser light sources.
Typically, fixed air gap, non-plug contacting or high density filter elements have been used in optical transmission systems and data links that use multimode-to-multimode or single mode-to-multimode connections. The latter case uses the multimode fiber at the detector in a single mode system as a photon bucket. Although systems of less than 1.0 Gb/s are not typically affected adversely by high reflected power, in some cases, systems of less than 1.0 Gb/s, such as, for example, in two-way transmission on a fiber, can be adversely affected.
Multiple reflections from two or more connections may cause system degradation which is referred to as multiple path interference (MPI). MPI is a phenomenon well known in classical optics and is realized whenever there are two or more optical discontinuities. The two major mechanisms that cause optical discontinuties are connections which are less than ideal and air gap attenuators.
Reflections reduce the signal-to-noise ratio of a receiver by two effects. First, multiple patterns from interferometric cavities that feed back into the transmitter can cause a conversion of a laser's phase noise into intensity noise. The receiver picks up the degraded signal. Also, multiple paths can introduce spurious "ghost signals", which arrive at the detector within variable delays, thereby producing intersymbol interference. Both effects result in an effective power penalty of several dB at the receiver. Inasmuch as these effects are signal dependent, increasing the transmitted power does not improve the error performance. Bit-error rate floors have been observed in laboratory gigabit/second fiber transmission systems due to multiple reflections from connectors and splices.
Reflections occur at a glass-air interface because of the difference in the refractive indices of the two materials. Each optical fiber with its end face cleaved perpendicularly to the fiber axis reflects at about a 3.5% level. When optical fiber ends are polished, the refractive index increases for a thin surface layer whereupon the reflectance can increase to over 5.5%.
Two surfaces such as the end surfaces of two spliced optical fibers form a cavity within which multiple reflections can occur. When the distance between the end faces equals an integral number of half wavelengths of the transmitter wavelength, all round trip distances equal an integral number of in-phase wavelengths and constructive interference occurs. This cause a quadrupling of reflectance to about 14% for unpolished end faces and to over 22% for polished end faces. On the other hand, a quarter wavelength displacement of the surfaces leads to constructive interference and no reflection.
One way of reducing reflective effects at a transmitter is to use an optical isolator which prevents light from reentering the laser. However, the use of an isolator results in some additional forward transmission loss and possible polarization effects.
Reflectances of components also can be reduced by using an index matching oil or gel between interfaces. Perfect matching is not likely because of the difficulty in matching the complex refractive index profile of the optical fiber, contamination from airborne dust, and because of temperature effects on the index material. Connectors which provide for contacting end faces can be used, if care is taken not to damage the end faces during installation or service. Also, anti-reflective coatings can be applied to ends of fibers, but both plugs must be coated, requiring replacement of existing plugs in pairs.
What is needed and what has not been provided by the prior art is an in-line, low-reflection attenuator for ferrule connectors such as the ST or FC connector which overcomes the foregoing problems and which may be used, for example, in single mode-to-single mode connector arrangements. The sought-after attenuator is required for high speed lightwave transmission systems with distributed feedback lasers, and amplitude modulated cable television transmission where unwanted reflections in the network can result in optical feedback into the laser causing laser instability and receiver noise. Also, the sought-after low reflection attenuator is needed to minimize systems degradations due to multiple path interference. The sought after in-line attenuator must be structured keeping in mind that the level of reflected power can be affected adversely by a mismatch in index of refraction in the transmission path, by the length of the gap between optical fiber ends, by laser linewidth, by frequency and by the distance between the two connections.
What is sought and what does not appear to be available in the prior art is a ferrule connector arrangement comprising an in-line attenuator which results in low reflected power. Desirably, the sought after attenuator may be integrated easily and be compatible with existing ST and FC connection systems.